The discovery of structural phase transitions in a series of alkanenitriles, acetonitrile (CH3CN), propionitrile (CH3CH2CN), nonadecanitrile [CD3(CH2)(19)CN], but not butyronitrile (CH3CH2CH2CN), at the air/water is described. Using sum frequency spectroscopy, the phase transition is manifested by an abrupt change in the orientation and an abrupt change in the vibrational frequency of the CN head group of the interface nitrile molecules. The competition between the distance dependent nitrile-nitrile dipolar interactions and the hydrogen bonding and solvation of the CN moieties is used to describe the phase transitions. A new application of second harmonic and sum frequency generation to study the interfaces of centrosymmetric microscopic particles is presented. Examples of this new use to the adsorption of molecules to a polystyrene microsphere/aqueous interface and the polarization of bulk water molecules by a charged microsphere of polystyrene sulfate are discussed.